The study was conducted at the experimental plots of the Laboratory of Floriculture and Landscape Architecture at the Agricultural University of Athens (lat 37° 59′ N, long. 23° 42′ 41″ E) from 20 March 2010 until 20 September 2012. The study comprised of 40 outdoor experimental plots, each of which was 60 cm × 58 cm. The experimental site was covered with perforated plastic liner and was levelled with the use of 200 mm coarse gravel bed. The experimental plots were constructed on top of the gravel shaft with wooden boards 20 mm thick × 200 mm wide. The wooden boards were utilised to separate adjacent plots and to prevent mechanical and hydrological continuity between them. The different substrate depth (either 7.5 or 15 cm) was accomplished by increasing or reducing the height of the gravel within each experimental plot.